Haulage vehicles are used in many diverse applications. For example, they are used to haul waste in urban areas and also to move earthen material in an environment such as an open-pit mine. The use of haulage vehicles in these types of environments typically require special construction considerations, making the vehicle the largest cost item in the operation. Replacement of the vehicles, which is necessary from time to time, involves large expenditures of capital. These and other factors have led vehicle fleet operators to search for ways to improve the effectiveness of the vehicles in order to lower costs and maintain a profitable operation in the face of declining markets and increasing, world-wide competition.
Often, haulage vehicles are subjected during their routine use to loads which differ greatly in weight because of different material density and/or the ability of some material to more tightly pack when loaded into the body of the vehicle. As a result, vehicle bodies which are generally filled to their full volume capacity may carry weight loads which exceed the weight capacity of the vehicle. Repeated occurrences of overloading result in the premature deterioration of the structural integrity of the vehicle, thus requiring repair or replacement of parts before anticipated. In order to avoid the damage caused by overloading, the vehicle body can be filled to a volume which assures the vehicle is not overloaded even for the most dense material. Although underloading may prevent the premature deterioration of the structural integrity of the vehicle, it sacrifices the vehicle's load-hauling efficiency. Therefore, a vehicle which is expensive to operate becomes even more expensive to operate when it is underloaded. Accordingly, there is a need to precisely measure the load carried by a haulage vehicle. This need has stimulated the development of on-board weighing devices that monitor and measure the vehicle's load.
Of course, in order to measure the on-board weight of a load carried by a vehicle, the vehicle must necessarily incorporate load sensors into its frame and/or body. In a dump-body vehicle, the body is movable on the frame of the vehicle between lowered and raised positions. To provide for this movement, the body is usually attached to the frame only by a pair of hinge assemblies and a pair of hydraulic cylinders.
In one approach to provide an on-board weighing device for a dump-body vehicle, load sensors are incorporated into the hinge assemblies and the hydraulic cylinders. Accordingly, in order to measure the load, the body must be lifted from its lowered position by the hydraulic cylinders so that the weight of the load is transferred to the frame through the cylinders and the hinge assemblies. Although the accuracy of the load measurements obtained from load sensors associated with the hydraulic cylinders and the hinge assemblies is satisfactory, the structural integrity of the vehicle may be degraded by modifications of the hinge assemblies and hydraulic cylinders required to incorporate the load sensors. Moreover, the impact of falling material onto the bed of the body is especially severe for the frame of the vehicle when the body is lifted slightly from its lowered position.
More important than the structural disadvantage of on-board weighing devices which incorporate load sensors in the hinge assemblies and hydraulic cylinders, is the disadvantage of requiring the body to be lifted off the frame in order to obtain a weight reading. Because this requirement consumes valuable time otherwise available for loading, hauling and unloading and because of the concentration of the load on the frame, the operator of the vehicle is discouraged from weighing the load; it is faster to approximate the load. Since the on-board weighing device interferes with an efficient and smooth hauling operation, there is a tendency to not use the weighing device. Therefore, the advantage of this on-board weighing device in dump-body vehicles has not been fully realized. Also, the requirement of lifting the body off the frame in order to obtain a weight measurement prevents continuous or periodic monitoring of the body's weight.
An approach to continuously monitoring and measuring the load carried by a haulage vehicle, is to use pressure gauges or similar type load sensors in the vehicle's suspension. Usually, in these types of weighing devices, the fluid pressure within a hydraulic suspension cylinder is sensed. Because of the relatively short stroke of the cylinder and the relatively large amount of frictional resistance to the cylinder's movement (the front cylinders normally also serve as the front axle spindles), the pressure reading is not a satisfactorily accurate indication of the vehicle's weight. In addition, the modification of the suspension to include load sensors opens the possibility of degrading the suspension system.
Because haulage vehicles often represent the bulk of the capital investment of a business utilizing such vehicles, their efficient management is of great importance. Even for small improvements in the efficiency of a vehicle and its fleet, significant savings can be realized. The lack of a convenient and accurate mechanism for weighing loads carried by a haulage vehicle has to some extent prevented such mechanisms from becoming important tools for effectively managing a vehicle. Management of individual haulage vehicles and vehicle fleets has historically been less than desired since management decisions have been based on inadequate information as to the day-to-day haulage performance of each vehicle.